Remotely-operated underwater or sub-sea vehicles, known in the industry as "ROVs," are in widespread use in connection with a variety of different underwater applications. One area in which these underwater vehicles or submarines are frequently employed is in the off-shore oil drilling industry. An ROV typically will be lowered on a tether cable off the drilling platform so that it can be operated through command signals sent down electrical conductors in the cable to permit remote viewing of the drill stack or ocean floor. Some ROVs also include mechanical manipulators which can be used to perform various underwater tasks associated with the drilling operation.
As will be appreciated, the deployment of an ROV in heavy seas or in a strong current, which are typically present in locations such as the North Sea, for example, can pose serious problems. Usually ROVs are approximately neutrally buoyant. Accordingly, the wave action and/or current can easily sweep the tether-operated submarine underneath the platform and into the drill stack with resultant damage to the vehicle and/or wrapping of the tether cable around the drill stack.
In order to avoid this problem, such vehicles are typically deployed in heavy sea environments from a so-called "garage." An ROV garage is a negatively buoyant framework in which the neutrally buoyant underwater vehicle is housed. The garage is lowered by a lowering cable from the drilling platform, and in addition to the ROV, the garage includes a powered reel or cable storage device with a bailer that permits paying out and reeling in of the ROV tether cable from the garage. This negatively buoyant garage assembly allows the ROV to be lowered down through the wave-action interface to the desired depth while keeping the lowering cable taut, at which point the ROV swims out of the garage and ROV cable is paid out of the garage by the powered reel to permit remote operation of the underwater vehicle.
Such garage-deployed ROVs have performed satisfactorily, but the garage tends to be a relatively complex and heavy structure that adds considerably to the overall cost of the system. In effect, two winches are provided, one on the platform to lower the garage by the lowering cable and one inside the garage to pay out the ROV cable. In addition, the controls become more complex and the strength of the lower cable between the garage and the platform-mounted winch must be strong enough to carry the substantial weight of the garage and ROV.